1. Field of the Invention
The present invention relates generally to a method of selecting/reselecting a cell in a mobile station, and in particular, to a method of efficiently reselecting a cell in a mobile station using Global Positioning System (GPS).
2. Description of the Related Art
Commonly, when a mobile station is turned on, the mobile station searches for a cell of a dedicated Public Land Mobile Network (PLMN), selects the cell as a serving cell, negotiates a channel, and accesses the PLMN. While the mobile station is maintaining a power-on state, cell selection and reselection is continuously performed. In particular, when the mobile station moves from a serving cell to another cell, the mobile station reselects a cell based on signal quality instead of the serving cell getting away from the mobile station. This cell reselection process of the mobile station includes the step of selecting a more proper and related Radio Access Technology (RAT) based on idle mode electronic wave measurement and cell reselection criteria. As described above, according to the conventional cell reselection method, a mobile station reselects an adjacent cell satisfying the cell reselection criteria from among cells suitable for the mobile station.
The cell reselection process will now be described with reference to FIG. 1. FIG. 1 is a diagram of a conventional mobile communication system. Referring to FIG. 1, a mobile communication system, such as Global System for Mobile Communication (GSM)/General Packet Radio Service (GPRS), includes a plurality of cells 102 to 112, each defining a wireless coverage area established by a fixed site base station system (BSS) located in the corresponding cell. For example, a cell 102 defines a wireless coverage area established by a base station system (BSS) 114, and similarly, each of the other cells 104 to 112 defines a wireless coverage area established by a BSS (not shown) located in the corresponding cell.
When a mobile station (MS) 100 moves from a location X of the mobile communication system to a location Y, the MS 100 continuously monitors signal characteristics from the BSSs of the cells 102 to 112 and selects a cell based on various selection criteria. For example, if a cell 110 is selected as an optimal coverage area according to a signal characteristic from the cell 110 based on the cell reselection criteria, the cell 110 is considered as a serving cell or a cell to/from which the MS 100 transmits/receives packet data.
As described above, the MS 100 continuously monitors signal characteristics from the cells 102 to 112. For example, as the MS 100 moves from the location X to the location Y along the path marked by the dashed line, the MS 100 moves from the coverage area of serving cell 110 to the coverage areas of the other cells, e.g., the cells 112 and 106. When the MS 110 determines another cell, e.g., the cell 112, as an optimal cell according to a signal characteristic from the cell 112, the MS 110 reselects the cell 112 as the serving cell. When the MS 110 determines another cell, e.g., the cell 106, as an optimal cell according to a signal characteristic from the cell 106, the MS 110 reselects the cell 106 as the serving cell.
In more detail, if the MS 100 moves from the cell 110 corresponding to the location X to the cell 106 corresponding to the location Y, the MS 100 calculates a C1 value, which is a path loss criterion parameter, a C2 value, which is a reselection criterion parameter, a C4 value, which is a signal strength threshold criterion parameter, a C31 value, which is a signal level threshold criterion parameter, and a C32 value, which is a cell ranking criterion parameter. As a result of the calculations, it is determined that the cell strength is good in cell 112 and cell 106, and the cells 102 and 112 are improper cells in which the service provided by a current terminal cannot be maintained. When the MS 100 moves from the cell 110, which is the serving cell, to the cell 106, the cell 112, which is an improper cell, has the priority of the cell reselection according to the cell strength to receive. Then, the MS 100 determines whether the cell 112 is a proper cell by receiving cell information including whether it is proper or not from the cell 112. As a result of the determination, if the MS 100 determines that the cell 112 is an improper cell, the MS 100 receives cell information indicating whether it is proper or not from the cell 106, which has the next priority of the cell selection according to the cell strength to receive, and determines whether the cell 106 is a proper cell. As a result of the determination, if the MS 100 determines that the cell 106 is a proper cell, the MS 100 camps on the cell 106.
As described above, an MS camped on a serving cell in a mobile communication system performs cell reselection based on the cell reselection criteria. The cell reselection criteria by which the MS attempts cell reselection will now be described. A first cell reselection criterion is where the C1 value, which is the path loss criterion parameter, is greater than a set threshold in the MS camped on the serving cell, and in this case, the cell reselection is performed. A second cell reselection criterion is a downlink signaling failure. A third cell reselection criterion is where a better cell is discovered in the same location area based on the C2 value, the C31 value, and the C32 value. A fourth cell reselection criterion is where a far better cell is discovered in another location area.
In the conventional cell reselection process, if an adjacent cell satisfying the cell reselection criteria described above is discovered, an MS receives from a network cell information including whether the adjacent cell is proper. Thereafter, the MS determines whether the adjacent cell is proper based on the received cell information, and if the MS determines that the adjacent cell is proper, the MS performs the cell reselection for the adjacent cell. If the MS determines that the adjacent cell is improper, the MS performs the same process again for a cell having the next priority according to the cell strength. As a result of the determination, if the MS determines that the cell is a proper cell, the MS camps on the cell.
In detail, if an MS providing the conventional GSM/GPRS function cannot receive a relevant service in a current serving cell in an idle state, a cell reselection process of searching for a new cell in which the service can be provided is necessary. To do this, when the MS searches for a proper cell, the MS calculates the C1, C2, C4, C31, and C32 values based on periodically measured transmission power values of BSSs in adjacent cells and its own transmission power value. When the MS searches for the most proper cell at the time of the cell reselection, the MS selects a new cell by which the service can be maintained by determining whether the new cell is a proper cell based on the calculated values.